A light-emitting diode (LED) array consists of multiple P-N or P-I-N junction LEDS fabricated on a single substrate. An advantage of a light-emitting diode array is that it can be used to process image information with relative ease, by electrically controlling the discrete diodes. Because of this, light-emitting diode arrays are being improved and applied in a variety of ways.
One example relates the use of printers as hard-copy data output devices. With the increasing importance of electronic information in today's world, printers need to be able to print faster and at higher densities in order to cope not only with the growing amounts of information, but also with the inclusion of image information in the form of graphs, drawings, photographs and the like. One way of achieving this is to use light-emitting diode arrays as the light sources in printers.
Laser printers, which employ a laser light source, and LED printers, which employ an LED array as the light source, are two examples of non-impact optical printers. A laser printer requires the use of a mechanical mechanism such as a rotating polygonal mirror for the scanning laser beam, and a corresponding complex optical system. An LED printer, on the other hand, only requires that the light-emitting diode array are controlled to switch on and off. As an LED printer is therefore structurally straightforward requiring no mechanical moving parts, using instead an optically magnifying lens array, it is possible for LED printers to be smaller, faster and more reliable than laser printers.
FIG. 4 is a cross-sectional illustration of a conventional homojunction type LED array 8 used in an LED printer. For simplicity only two light-emitting diodes (hereinafter also referred to as light-emitting elements) are shown. With reference to the figure, the array 8 comprises a substrate 10 of n-type conductivity GaAs having a layer 12 of n-type conductivity GaAsP of about 15 microns thick on a surface thereof. Spaced apart zinc doped regions 16 of p-type conductivity and about 1.5 microns deep are in the layer 12. On the surface of the GaAsP layer 12 is a masking layer 14 of SiN.sub.x which has openings 15 therethrough over the p-type conductivity regions 16. P-electrodes 18 are on the p-type conductivity regions 16 and an n-electrode 20 is on the surface of the substrate 10 opposite the GaAs layer 12. An antireflection layer 22 of SiN.sub.x covers the masking layer 14 and the p-type regions 16. The antireflection layer 22 is removed from the non-light-emitting element portions to form a p-electrode 18 bonding pad. In the array 8, the light-emitting element is formed by the P-N junction at the interface between the n-type conductivity GaAsP layer 12 and the p-type conductivity regions 16.
The array 8 is made by epitaxially depositing the n-type conductivity GaAsP layer 12 on the substrate 10 by the use of vapor-phase epitaxy (VPE). The masking layer 14 is then deposited on the GaAsP layer 12 and is provided with the openings 15 using standard photolithographic and etching techniques. Zinc is then diffused into the GaAsP layer 12 through the openings 15 in the masking layer 14 to form the p-type regions 16. The electrodes 18 and 20 are then deposited on the p-type conductivity regions 15 and the substrate 10 followed by the deposition of the antireflection layer 22.
Two problems encountered when such a light-emitting diode array is used in a printer, unlike when individual LEDs are used, are crosstalk between adjacent light-emitting elements, and variation in characteristics from element to element.
In the type of conventional light-emitting diode array 8 shown in FIG. 4, the n-type conductivity GaAsP layer 12 has a high internal absorption index which is utilized to prevent crosstalk between adjacent elements, while variation in the characteristics arising non-uniformities in the fabrication process is reduced by using only selective diffusion in forming the light-emitting elements.
However, the light emitting diode array thus formed contains numerous lattice defects owing to a lack of lattice matching between the n-type conductivity GaAsP layer 12 used as the light-emitting material and the n-type conductivity GaAs substrate 10. As a result there is considerable non-conformity of the material itself, so the emission efficiency is low. In addition, because the P-N junction is a homojunction with a very low injection efficiency, it is difficult to improve the emission efficiency.
The AlGaAs single heterojunction type light-emitting diode array 28 shown in FIG. 5 was developed previously to overcome the drawbacks of the conventional GaAsP light-emitting diode array.
With reference to FIG. 5, the array 28 comprises a substrate 30 of p-type conductivity GaAs having a layer 32 of p-type conductivity Al.sub.x Ga.sub.1-x As on a surface thereof. The layer 32 is 10 microns thick and is doped with zinc to a concentration of 5.times.10.sup.17 impurities/cm.sup.3. On the layer 32 is a layer 34 of n-type conductivity Al.sub.y Ga.sub.1-x As which is 5 microns thick and doped with tellurium to a concentration of 8.times.10.sup.17 impurities/cm.sup.3. A layer 36 of n+ type GaAs is on the layer 34. The layer 36 is 0.1 microns thick and is doped with tin to a concentration of 5.times.10.sup.18 impurities/cm.sup.3. For emitting light with a wavelength in the region of 720 nm, the aluminum composition is set at x=0.2 and y=0.5.
The p-type conductivity layer 32 and the n-type conductivity layer 34 are etched so that the layer 34 and a portion of the layer 32 forms spaced mesas 44, each of which defines a separate light-emitting diode. The layer 36 is on a portion of the layer 34 of each mesa 44 to form a contact layer for the diode. An n-electrode 38 is on the layer 34 on each mesa 44 and a p-electrode 40 is on the surface of the substrate 30 opposite the layer 32. An antireflection layer 42 of SiN.sub.x is over the mesas 44 and the exposed surface of the layer 32.
The array 28 is made by epitaxially depositing the layers 32, 34 and 36 in succession on the substrate 30 using liquid phase epitaxy (LPE). Then, n-electrodes 38 and p-electrode 40 are formed by deposition, and the unnecessary portions of the n-electrode 38 are removed by photolithography and plasma etching. The GaAs layer 36 is selectively etched so as to leave the n-electrode 38 portions. Photolithography and chemical etching are then used around the light-emitting regions so as to form the mesa-shaped light-emitting regions 44, with the etching extending about 1 micron into the layer 32. Plasma CV is then used to form the antireflection layer 42. This is followed by the use of heat treatment to form ohmic contacts for the n-electrode 38 and p-electrode 40, which completes the fabrication of the heterojunction light-emitting diode array 38.
Structurally, this type of heterojunction light-emitting diode array consists of discrete high-luminance LEDs arranged into a single array. The use of a heterojunction provides an improvement in the injection efficiency, and by using the n-type conductivity Al.sub.y Ga.sub.1-y As layer 34 which is transparent to the light emitted by the light-emitting p-type conductivity Al.sub.x Ga.sub.1-x As layer 32, energy attenuation caused by internal absorption is avoided, enabling an emission efficiency to be achieved that is several times higher than that achievable with the homojunction light-emitting diode array 8 of FIG. 4.
However, there are problems with the LED arrays described above. As mentioned, unlike when single, discrete LEDs are involved, in an array of LEDs consisting of a multiplicity of light-emitting elements closely arranged on a single substrate, it is important to reduce optical crosstalk between elements. For this, in the light-emitting diode array 28 of FIG. 5, the n-type conductivity Al.sub.y Ga.sub.1-y As layer 34 used as a transparent window has to be completely removed between elements by etching, and in addition non-mesa portions of the emission region have to be etched to a certain minimum depth to reduce optical bleeding.
It is known that the diffusion length of minority carrier electrons injected into the active layer from the heterojunction is in the order of 10 microns, meaning the active layer has to be etched to at least about 10 microns in order to reduce crosstalk and optical bleeding and attain optimum emission efficiency. However, owing to the difficulty of achieving uniform etching with a good level of reproducibility, the result has tended to be a degradation in characteristics.
In the case of single LEDs, by using a double heterojunction structure in which the p-type conductivity Al.sub.x Ga.sub.1-x As layer 32, the active layer, is one micron thick of thinner, under which is formed a p-type conductivity Al.sub.y Ga.sub.1-y As layer with a large Al mixed crystal ratio, the injected carriers can be efficiently confined in the small-energy-gap p-type conductivity Al.sub.x Ga.sub.1-x As layer 32, considerably reducing internal light absorption and increasing overall efficiency. However, although this might improve the overall emission efficiency, the high refractive index of the active layer means that most of the light is not emitted but reflected, resulting in a very low efficiency in the order of just several percent. Thus, such a light-emitting diode array is not a solution.